I built my PC a year ago and was workign fine. All of a sudden it is not posting or doing anything at all:
1) I can power it up, the fans and lights will come on, but no video signal, no beeps and no HDD spinning.
2) I can not turn it off with the power switch, neet to turn of the PS.
3) The DVD tray will not open, but the DVD light comes on and stay on.

Here is what I have done.
a) Checked all voltages on PS (with no load) and all were in spec.
b) If I remove the data cable from DVD, and just keep the power attached, then DVD tray will open
c) I have swapped the MOBO with an identical (Gigabyte 770-UD3 with no on board video) and same exact thing.

Will be grateful for any input.

First of all check all your connections to make sure they are secure.

When you tried the new mobo did you use your current cpu?

Try turning it on with only the cpu mobo ram and power supply intact. Leave in 1 ram stick instead of 2, try it with each stick in while its stripped down. Disconnect everything else including your mouse, keyboard, cd rom etc. Disconnect and reconnect the left over cables. Does it give you a beep code? If it does not give you the normal 1 short beep then you know it is one of these 4 parts. With the other mobo in mind that you swapped out you then narrowed it down to 3 parts. If the problem does not occur after doing this add 1 part at a time until you find the bad part.

Furthermore you can find a store that will let you buy and return a power supply for a cash refund, swap out the power supply to see if this is the cause, if not return it. Rinse and repeat with ram and a cpu if possible.

There is a butt load of info in the guides section in the forums.

Read the various NO POST guides in the guides forum by me and others.

Solved

Thanks everyone for replies. Turned out it was PS afterall.

Take home lesson: Testing Power supply under no load is not very useful.